1. Field of Invention
This invention relates in general to radio frequency (RF) transceivers and more particularly to mixers used in RF transceivers that provide DC offset calibration.
2. Related Art
An RF transceiver (transmitter/receiver) typically requires a frequency translation function, in which a signal is translated from one carrier frequency, ω1, to another frequency, ω2. The translation is carried out by what is known as a “mixer.” In effect, a mixer translates the center frequency, ω1, of a signal band, to a center frequency, ω2, by mixing the signal band with a local oscillator signal having a frequency of ωLO, where ωLO=(ω1−ω2. The mixing operation yields a first band centered around ω2 and a second band centered around 2ω1−ω2. An appropriate filter is then typically employed to select the desired signal band centered around ω2. When ω1>ω2, this operation is known in the art as downconversion mixing. On the other hand, when ω1<ω2, the operation is known as upconversion mixing.
Crols and M. S. J. Steyaert, “A 1.5 GHz Highly Linear CMOS Downconversion Mixer”, IEEE J Solid-State Circuits, vol. 30, pp. 736-742, July 1995 disclose a mixer design in which MOS transistors, operating in their linear region, are used as voltage-dependent resistors to modulate a signal. The impedance of the transistors in the linear region is controlled by the input signal to be modulated. Local oscillator signals are provided to the source/drain side of the transistors. There are certain weaknesses with this design, however. First, the linearity of the mixer is limited in that the impedance of the MOS transistors in the linear region become non-linear when the gate control voltage varies considerably. Second, the gain of the mixer is not well controlled, since it is difficult to match the impedance of the MOS transistors in the linear region to that of the resistor in the feedback loop of the amplifier. Finally, even if additional sets of MOS transistors are included for the DC offset cancellation path, mismatch of the transistors can be of a concern, especially if different signal path gains are to be implemented.
A typical transceiver is subject to other known non-idealities, which can yield an undesirable output. One such non-ideality relates to a direct current (DC) component, which is often introduced into the input signal of an upconversion mixer. If the desired input signal is centered around DC, the extra DC component introduced into the input signal will corrupt the signal and degrade the signal quality. The DC component, after mixing with the local oscillator (LO), generates an undesired tone at ωLO at the output of the mixer.
Another example of non-ideality relates to LO feedthrough and self-mixing in a downconversion mixer, in which the LO signal is coupled into the input signal and subsequently is mixed with the LO itself in the mixer, producing an undesirable DC component.